username works but password that has never been changed no longer is accepted
PASSWORD NOT ADDEPTED, X-MARKS ADDED. CAN'T UNINSTALL IT
You installed Xmarks but it is not accepting your password? You would need to contact Xmarks support.
About your other problem:
1. On the menu bar, select Tools
3. Select the add-on you want to uninstall.
4. Click Uninstall . Again when prompted
You will need to restart Firefox
If the above failed to work see Cannot remove an add-on (extension or theme)